Measurement
[2] However, in other fields such as statistics as well as the social and behavioural sciences, measurements can have multiple levels, which would include nominal, ordinal, interval and ratio scales.
Since the 18th century, developments progressed towards unifying, widely accepted standards that resulted in the modern International System of Units (SI).
The system defines seven fundamental units: kilogram, metre, candela, second, ampere, kelvin, and mole.
Artifact-free definitions fix measurements at an exact value related to a physical constant or other invariable phenomena in nature, in contrast to standard artifacts which are subject to deterioration or destruction.
Over the course of human history, however, first for convenience and then for necessity, standards of measurement evolved so that communities would have certain common benchmarks.
Units of measurement are generally defined on a scientific basis, overseen by governmental or independent agencies, and established in international treaties, pre-eminent of which is the General Conference on Weights and Measures (CGPM), established in 1875 by the Metre Convention, overseeing the International System of Units (SI).
For example, the metre was redefined in 1983 by the CGPM in terms of the speed of light, the kilogram was redefined in 2019 in terms of the Planck constant and the international yard was defined in 1960 by the governments of the United States, United Kingdom, Australia and South Africa as being exactly 0.9144 metres.
Many Imperial units remain in use in Britain, which has officially switched to the SI system—with a few exceptions such as road signs, which are still in miles.
Many people measure their height in feet and inches and their weight in stone and pounds, to give just a few examples.
Imperial units are used in many other places, for example, in many Commonwealth countries that are considered metricated, land area is measured in acres and floor space in square feet, particularly for commercial transactions (rather than government statistics).
Metric units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes.
Weight, on the other hand, refers to the downward force produced when a mass is in a gravitational field.
In free fall, (no net gravitational forces) objects lack weight but retain their mass.
A spring scale measures force but not mass, a balance compares weight, both require a gravitational field to operate.
Some of the most accurate instruments for measuring weight or mass are based on load cells with a digital read-out, but require a gravitational field to function and would not work in free fall.
[11] In substantive survey research, measurement error can lead to biased conclusions and wrongly estimated effects.
For example, consider the problem of measuring the time it takes an object to fall a distance of one metre (about 39 in).
In the classical definition, which is standard throughout the physical sciences, measurement is the determination or estimation of ratios of quantities.
The classical concept of quantity can be traced back to John Wallis and Isaac Newton, and was foreshadowed in Euclid's Elements.
In weaker forms of representational theory, such as that implicit within the work of Stanley Smith Stevens,[16] numbers need only be assigned according to a rule.
